George MacDonald Fraser at home on the Isle of Man. His first novel is ‘not vintage Fraser’, say his family.
Photograph: Terry Smith/Time & Life Pictures/Getty

Scottish author George MacDonald Fraser’s famous Flashman Papers, which transformed Tom Brown’s best known bully, Harry Flashman, into an unlikely Victorian hero, have won legions of fans and accolades.

But the novelist’s first attempt at fiction would prove less of a literary success, failing to find a publisher – until now.

Seven years after his death, a forgotten novel, written 60 years ago and found locked away in a fireproof safe in his old study, is finally to make it into print. Vintage Fraser it is not, the author’s family candidly admit, though “Captain in Calico”, about real-life pirate John “Calico Jack” Rackham, is still a “cracking little story”.

“Captain in Calico would probably be even less likely to find a publisher today than 60 years ago, and we do not want readers to be deceived into thinking it is vintage George MacDonald Fraser, and of the standard of the Flashman novels, or the McAuslan short stories,” said the author’s daughter, Caro Fraser.

“Indeed, we thought long and hard before allowing it to be published, and are only doing so because we believe that, as an early work, Captain in Calico is a delightful curiosity, one which we hope will provide fans of GMF with a fascinating insight into the inspirations and creative impulses that turned him into such a fine novelist.”

The manuscript was found by Fraser’s three children when they cleared out their parents’ home following the death of their mother. They were eventually persuaded to publish by HarperCollins, their father’s publisher.

The book is based on the life of Rackham, captain of the pirate sloop Revenge, who with his flame-haired Irish-born lover, Anne Bonny, operated in the Bahamas and in Cuba. He was hanged after being captured by a pirate hunter.

Nicknamed after his trademark colourful calico clothing, Rackham is chiefly remembered for the design of his Jolly Roger flag, a skull with crossed swords, and for having two female crew, Bonny and Mary Read. He features in Fraser’s 1983 novel, The Pyrates, and a skeleton said to be his is visible in the Hollywood box-office hit Pirates of the Caribbean: The Curse of the Black Pearl, hanging outside Port Royal alongside the sign “Pirates, ye be warned”.

Captain in Calico features the unscrupulous Rackham when he returns from the high seas to ask the governor of the Bahamas, Woodes Rogers, for a royal pardon, and the governor sees his chance to put his own devious plans into action. “Their agreement sets off an adventure of betrayals, counter-betrayals, plots and escapes that see Rackham join forces with the scheming but seductively beautiful pirate Anne Bonny,” said the publisher.

Full of the influences of the writers he loved as a boy - Rafael Sabatini, PC Wren, GA Henty, Sir Walter Scott – the book demonstrates “the author’s early flair for writing scoundrels of the highest order,” the publisher added.

The book will be published on 10 September, but throughout the whole of August will be available exclusively through Fraser’s favourite London bookshop, Heywood Hill, in Mayfair, which was responsible for the sale of his library in 2014.

George MacDonald Fraser’s Flashman series.

Fraser, who died aged 82, was a one-time journalist who found fame with his novels looking back on the exploits of the fictional coward and bully from Tom Hughes’s Tom Brown’s School Days. The 12 books, written as memoirs, follow the cowardly, deceitful but charming Flashman’s escapades as an unlikely hero of the British army during the 19th century. In addition to his novels, Fraser also wrote numerous screenplays, including The Three Musketeers and the James Bond film Octopussy.

On discovering the manuscript, his daughter said: “We were clearing out my parents’ house following my mother’s death, and in the very last room, right in the corner, in what had been my father’s old study, was a fireproof safe which we had forgotten about. It took us a while to get the combination. And in there was this manuscript which he’d clearly put away hoping, I think, it would be found.

“If he had plans for another Flashman novel, he didn’t leave any trace behind, because I don’t think he wanted anyone to use material that he had half-prepared,” she told BBC Radio 4’s Today programme. “But this was on its own, and in the safe.”